This longitudinal study which was initiated in 1995 to evaluate the validity of DSM-IV attention-deficit/hyperactivity disorder (ADHD) and its subtypes among 4 to 6 year old children. To date, 130 children who met DSM-IV criteria for ADHD in Wave 1 and 126 well-matched comparison children have been assessed in three annual waves (Waves 4 and 5 are ongoing). Analyses of Wave 1 data suggest that DSM-IV ADHD is valid in the sense of concurrent association with impairment in multiple domains. Across the first three annual waves, the overall diagnosis of DSM-IV has been highly stable. An unexpectedly high proportion of the ADHD probands have shifted from meeting criteria for one subtype of ADHD to another subtype across waves, however, raising serious concerns about the validity of the subtypes in this age range. The goals of this study during the next five years are to: 1) evaluate the predictive validity of DSM-IV ADHD for young children in terms of future functional impairment; 2) to continue to assess the temporal stability of the symptoms and subtypes of DSM-IV ADHD to determine if the subtypes stabilize during any part of childhood; 3) to evaluate two alternative approaches to subtyping ADHD from a longitudinal perspective; 4) to examine the influence of parenting on the outcomes of young children with ADHD; and 5) to study the developmental transition to childhood-onset conduct disorder among young children with ADHD. This proposed continuation of the present longitudinal study through 12 to 14 years of age would provide currently unavailable information on the predictive validity, temporal stability, developmental course, and outcomes of DSM-IV ADHD when it is diagnosed in young children. This information would be of great importance to a developmental understanding of ADHD and could influence future nomenclatures.